This invention relates to the field of raster distortion correction systems with switchable digital interpolating filters.
Raster distortion correction, for example convergence and pincushion correction, is an important aspect of television performance, particularly for large direct view and projection television receivers and monitors. Projection television receivers can present a very difficult challenge to distortion correction systems due to the off-axis orientation of two of the three projection tubes and due to the need to provide separate distortion control systems for each projection tube. In projection television receivers, the green tube is usually in a central orientation. After the raster of the green tube is corrected, the rasters generated by the red and blue tubes must be corrected and converged to match the raster generated by the green tube.
Generally, distortion correction data can be stored in a digital memory, read out of memory, processed by an interpolator to provide additional correction data, converted to analog form, analog low-pass filtered and amplified for use as a convergence correction deflection signal. Each distortion correction circuit must be optimized not only for each projection tube's internal geometry and mounting orientation, as well as the screen size and screen orientation, but for the horizontal scanning frequency of the input video signal as well. The analog low-pass filter, which can form an input for a preamplifier, is a part of the circuit that can be most sensitive to differences in horizontal scanning frequency. Moreover, many receivers presenting significant distortion correction challenges are already adapted to operate at the standard horizontal scanning frequency (1fH) and twice the standard horizontal scanning frequency (2fH). In fact, such receivers will also need to process video signals having a horizontal scanning frequency three times (3fH) the standard frequency.
If the distortion correction system operates with high horizontal frequency (for example 2fH or 3fH, or more) and very short retrace times, the frequency of the digital filter, and thus also the frequency of the D/A converters, is very high during horizontal retrace. This can lead to exceeding the permissible maximum frequency of the D/A converters. It is desirable to prevent exceeding the permissible maximum frequency.
Digital filters, particularly finite impulse response (FIR) filters, are sensitive to noise and signal transients that occur during horizontal retrace because FIR filters rely on earlier samples to generate output values. Even if the maximum frequency of the D/A converters is not exceeded, operation of the D/A converters at high scanning frequencies can cause large transient signals that can disturb proper operation of the digital filter at the beginning of horizontal trace. It is desirable to prevent the digital filters from disturbance due to transient signals occurring during horizontal retrace.